The present invention relates to an adhesive composition for bonding two or more different members through a fitting structure, a bonding method using the composition, and a composite member made by the bonding method. More particularly, it relates to an adhesive composition for making a composite member comprising two or more different members bonded through a fitting structure which is controlled in its expansion coefficient and residual stress, a method for bonding two or more different members using the composition, and a composite member made by the bonding method.
There is a method of using a solder for bonding different members, for example, a ceramic member and a metallic member. However, during cooling operation after bonding them at high temperatures, thermal stress is generated due to the difference in thermal expansion coefficients between the different members or between the member and the solder used for bonding of these different members to cause separation at the bonded interface. If one of the members is fragile, cracks occur in the vicinity of the bonded interface and sometimes the desired bonding strength or airtightness cannot be obtained. The products in which these defects are caused during the production steps must be disposed of as rejected products, and this results in an increase of the production cost for these composite members. Moreover, if they are subjected to thermal cycles in use, the defects occur after use for a certain period to cause deterioration of reliability of the products.
When different members are bonded using a solder, a method is generally employed according to which the surface of the ceramic member to be bonded is plated with a metal such as Ni to ensure wetting between the ceramic member and the solder. Then these members are disposed opposite to each other with a suitable space, the solder is poured into the space, and the members are bonded. There is another method according to which an additive such as Ti, which can ensure wetting by forming a reactive layer of a nitride or an oxide on the surface of the ceramic member, is added to the solder, without carrying out the plating with a metal. However, these methods are not preferred because they are not enough to reduce the thermal stress, and cracks are often formed on the side of the ceramic member which is fragile against thermal stress or separation is caused at the bonded part to adversely affect various performances such as bonding strength and airtightness required for composite member.
As methods for relaxing the thermal stress, a method of using a metal low in thermal expansion coefficient as an intermediate material at the time of bonding and a method of using as an intermediate material a soft metal which is high in reactivity with ceramics and can relax the stress by plastic deformation are generally employed. However, these techniques also suffer from the problems caused by difference in thermal expansion between the solder and the member, for example, low thermal cycle characteristics, and cannot necessarily be said to be high in general-purpose properties. Furthermore, there is a high-pressure solid phase bonding method which is now under development, but this method has unsolved problems for practical utilization and cannot provide a sufficient bonding strength.
On the other hand, as a composite solder, JP-A-6-126479 discloses a mixture of powders comprising materials having a higher melting point than the solder used for bonding a semiconductor chip and a substrate. However, this aims to solve the problem of insufficient wetting of the conventional composite solder caused by the powder also present on the surface by filling a powder comprising materials higher in melting point than the solder only in the central portion of the solder per se. In other words, this aims to increase the bonding strength at the bonded interface. This composite solder is not effective for decreasing thermal stress, and, hence, is not effective in solving the problems caused by the thermal stress between the members to be bonded or between the member and the solder.
As a result of intensive research conducted by the inventors for attaining the above object, they have found an adhesive composition for bonding at least two different members which does not cause decrease of bonding strength in the vicinity of the bonded interface due to thermal stress generated during cooling operation after bonding at high temperatures and does not cause generation of cracks in the member weak against the thermal stress during the cooling operation while maintaining an appropriate bonding strength between the different members. A Japanese patent application No.10-52971 was filed on this subject.
That is, as a result of intensive research, the inventors have found that at least two different members can be bonded without causing a decrease in the bonding strength in the vicinity of the bonded interface due to thermal stress generated during cooling operation after bonding at high temperatures and without causing generation of cracks in the member weak against the thermal stress during the cooling operation with maintaining an appropriate bonding strength between the different members by using as a base a hard solder which is not restricted by the kind of the members or shape of the members and which leaves room for selection of bonding shape and by adding to the hard solder a fine particle material which lowers the thermal stress. Thus, the above patent application was filed.
As a method for bonding with the above composition, the patent application discloses a method which comprises disposing two or more different members differing in thermal stress so that they are opposed with a space enough to bond them, and pouring the composition into the space or filling a given amount of ceramic or cermet fine particles and subsequently pouring a given amount of the hard solder in molten state, followed by cooling to produce a composite member. However, the composition cannot be used as it is for bonding the members without a sufficient space provided therebetween to be able to pour a given amount of the hard solder.
In the case of bonding two or more different members which must be bonded through a fitting structure, especially, in the case of bonding the members with selecting a very narrow clearance of about 0.01-0.30 mm, various troubles often occur unless a solder is filled also on the side of the members as uniformly as possible. On the other hand, when the clearance is greater than the above upper limit, cracks sometimes occur due to the residual stress which is generated due to non-uniform filling of the solder in the clearance and is contained in the solder at the time of melting of the solder. This will be more specifically explained.
For example, when a member having a columnar dented portion and a member having a columnar protruded portion are fitted, if the solder to be uniformly filled in the columnar clearance formed by these members is one-sidedly filled on only one side because the clearance has a room to some extent, the balance of thermal shrinkage stress is lost in the process of cooling and solidifying the solder and the member having the columnar protruded portion is pulled to one direction, resulting in residual stress and cracks. Moreover, there are demands for dimensional accuracy and beautiful appearance of the composite member as a finished product. Thus, since it is substantially impossible to pour a given amount of a hard solder from a narrow clearance in the members having only such a narrow clearance, these different members cannot be bonded by the above method. The term xe2x80x9cclearancexe2x80x9d here means a width of the space present between the wall surfaces of the different members in the fitting structure.
Under the circumstances, in an attempt to solve these problems, the inventors have found that a member having a dented portion which forms a fitting structure and a member having a protruded portion which forms a fitting structure and being different from the member having the dented portion can be bonded by fitting them to each other in the following manner.
A fine particle material is uniformly spread over the surface of the dented portion of the member having the dented portion, then a platy or powdery hard solder is disposed so as to cover at least a part of the layer comprising the fine particle material, further the member having the protruded portion is disposed, these are heated to a given temperature under application of pressure to melt the hard solder. This molten hard solder is penetrated into the layer comprising the fine particle material to form a bonding layer of an adhesive composition controlled in expansion coefficient and comprising the hard solder and the fine particle material, or a fine particle material is uniformly spread over the surface of the dented portion of the member having the dented portion. Then the member having the protruded portion having one or a plurality of holes in which a hard solder is inserted is disposed so as to closely contact with the layer comprising the fine particle material. These are heated to a given temperature under application of pressure to melt the hard solder, and this molten hard solder is penetrated into the layer comprising the fine particle material to form a bonding layer of an adhesive composition controlled in expansion coefficient and comprising the hard solder and the fine particle material.
An alternative is a member having a protruded portion at the end of which is formed a layer comprising a hard solder and a fine particle material is previously prepared, only a hard solder is disposed on the surface of a dented portion of a member having the dented portion, the member having the protruded portion at the end of which is formed a layer comprising the hard solder and the fine particle material is disposed on the hard solder disposed on the surface of the dented portion of the member, these are heated to a given temperature under application of pressure to melt the layer comprising the hard solder and the particulate material formed at the end of the member having the protruded portion and the hard solder disposed on the surface of the dented portion of the member having the dented portion, thereby to form a bonding layer of an adhesive composition controlled in expansion coefficient and comprising the hard solder and the fine particle material. Based on this finding, Japanese Patent Application No.11-180902 was filed on Jun. 25, 1999.
Furthermore, the Japanese Patent Application No.11-180902 filed on Jun. 25, 1999 proposes a composite member having improved thermal cycle characteristics and comprising two or more different members obtained by the above fitting and bonding method, characterized by comprising at least a member having a dented portion which forms a fitting structure and a member having a protruded portion which forms a fitting structure and being different from the member having the dented portion, said different members being fitted and bonded to each other with an adhesive composition controlled in expansion coefficient and comprising a fine particle material and a hard solder.
However, the dispersing material dispersed in the hard solder is subjected to plating with Ni in order to ensure wetting with the hard solder. The solder layer, which is a composite formed by this method, has a lower expansion coefficient than the hard solder, which is a metallic material, and is effective in inhibiting breakage of the ceramic, which is a material to be bonded, and in improving the thermal cycle characteristics of the bonded part. But when strength of the ceramic (which is the material to be bonded) is low, for example, in the case of aluminum nitride or the like, it is difficult to completely inhibit the breakage. And with increase of the area of the bonded part, the possibility of the occurrence of breakage further increases, and, besides, there is a problem that if the number of the bonded parts in one product is great, the percent defective of the products cannot be ignored. For the inhibition of breakage of the materials to be bonded, in addition to reduction of expansion coefficient of the solder, reduction of Young""s modulus and reduction of proof stress value are effective, but it has been found that positively advantageous operation for all of these physical characteristics is difficult using only these methods.
The object of the present invention is to provide an adhesive composition for bonding two or more different members which can give a bonded material excellent in heat resistance and other characteristics which inhibit breakage of the materials to be bonded by controlling the characteristics of the above-mentioned composite solder, namely, reducing the expansion coefficient, reducing the Young""s modulus and the proof stress value. A method for bonding two or more different members using the adhesive composition, and a composite member comprising two or more different members bonded by the above method is also provided.
As a result of intensive research conducted by the inventors for attaining the above object, it has been found that the above object can be attained by an adhesive composition for bonding two or more different members which comprises a hard solder and a mixture of two or more fine particle materials differing in wettability with the hard solder. As a result, the present invention has been accomplished.
Moreover, it has been found that a bonded material of a composite member comprising different members bonded through a fitting structure which is inhibited from breaking and excellent in heat resistance characteristics and others can be produced by using an adhesive composition controlled in expansion coefficient, Young""s modulus and proof stress value by the above-mentioned method. Thus, the present invention of the second aspect has been accomplished.
Furthermore, it has been found that a composite member excellent in heat resistance characteristic and others which comprises a member having a dented portion which forms a fitting structure and a member having a protruded portion which forms a fitting structure and being different from the member having the dented portion, the different members being fitted and bonded to each other with a bonding layer comprising a hard solder and a mixture of two or more fine particle materials differing in wettability with the hard solder, can be obtained without causing breakage by using the adhesive composition controlled in expansion coefficient, Young""s modulus and proof stress value. Thus, the present invention of the third aspect has been accomplished.